A current regulator is used to provide a constant current through a circuit element, such as a light emitting diode (LED). When the current regulator is arranged to provide a constant current through an LED it may be referred as an LED driver. In such applications the LED driver can maintain a constant current despite variations in a forward voltage of the LED. A particular type of LED driver is a flash LED driver that may be used to provide a high current through the LED for a short duration. This can be used to generate a bright flash for a camera.
FIG. 1(a) and FIG. 1(b) show schematics of prior art implementations of flash LED drivers. In FIG. 1(a) there is a LED current regulator 10, comprising a transistor 12, for providing a constant current to a LED coupled to an LED pin 14. A boost regulator is used to generate a supply voltage VOUT at a supply pin 16. The supply voltage VOUT is regulated such that a voltage at the LED pin 14 is the minimum voltage at which the transistor 12 can operate, whilst still providing a constant current. A forward voltage of the LED is a voltage difference between the supply pin 16 and the LED pin 14, which may be in the range of 2.9V to 4.5V. The supply voltage VOUT is dynamically adjusted based on the forward voltage of the LED and therefore power dissipation on the transistor 12 can be minimised. FIG. 1(b) shows a different implementation of a current regulator in the form of a low dropout (LDO) regulator 18. A boost regulator is used to generate a supply voltage VOUT at a supply pin 20.
A current regulator may be implemented in a system that uses a supply voltage that is shared with other functions. Another function may include the supply voltage being used for a USB On-The-Go (OTG) system. In such a system the supply voltage may vary and is susceptible to noise.